Preparation method of lithium-metal composite oxides

ABSTRACT

Disclosed is a method for preparing a lithium-metal composite oxide, the method comprising the steps of: (a) mixing an aqueous solution of one or more transition metal-containing precursor compounds with an alkalifying agent and a lithium precursor compound to precipitate hydroxides of the transition metals; (b) mixing the mixture of step (a) with water under supercritical or subcritical conditions to synthesize a lithium-metal composite oxide, and drying the lithium-metal composite oxide; and (c) subjecting the dried lithium-metal composite oxide either to calcination or to granulation and then calcination. Also disclosed are an electrode comprising the lithium-metal composite oxide, and an electrochemical device comprising the electrode. In the disclosed invention, a lithium-metal composite oxide synthesized based on the prior supercritical hydrothermal synthesis method is subjected either to calcination or to granulation and then calcination. Thus, unlike the prior dry calcination method or wet precipitation method, a uniform solid solution can be formed and the ordering of metals in the composite oxide can be improved. Accordingly, the lithium-metal composite oxide can show crystal stability and excellent electrochemical properties.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. §371of International Application No. PCT/KR2007/000851, filed Feb. 16, 2007,published in English, which claims priority from Korean PatentApplication No. 10-2006-0015849, filed Feb. 17, 2006. The disclosures ofsaid applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a novel method for preparing alithium-metal composite oxide having crystallographical stability andhigh-density characteristics. Also, it relates to an electrodecomprising the lithium-metal composite oxide prepared according to saidmethod, and an electrochemical device, which comprises said electrode,and thus shows high-capacity and long cycle life characteristics.

BACKGROUND ART

Recently, as the mobility and portability of electrical and electronicdevices have increased, the demand for secondary batteries has rapidlyincreased. Lithium secondary batteries started to be producedindustrially by Sony Corp., Japan, in the beginning of the 1990s, andoccupy the majority of the portable phone and notebook computer markets,because such lithium secondary batteries have advantages over priorNi—Cd and Ni-MH batteries in that they have light weight and highcapacity. Recently, such lithium secondary batteries have beenincreasingly used in high-output large-capacity batteries in electricpower tools, electric bicycles, electric scooters, game machines,wireless cleaners, service robots, hybrid vehicles, etc.

Lithium ion secondary batteries generally employ lithium cobaltate(LiCoO₂) as a cathode active material, carbon as an anode activematerial, and lithium hexafluorophosphate as an electrolyte. As thecathode active material, lithium cobaltate (LiCoO₂) and lithiumnickelate (LiNiO₂), having a layered structure, and lithium manganatehaving a spinel structure, are known, but lithium cobaltate is mostlyused in practice for commercial purposes. However, because not only thesupply and demand of cobalt as a main component is unstable, but alsothe cost of cobalt is high, materials obtained by partially substitutingcobalt with other transition metals such as Ni and Mn, or spinel lithiummanganate containing little or no cobalt, etc., started to becommercially used. Also, novel compounds showing high structuralstability even under high voltages, or materials obtained by doping orcoating existing cathode materials with other metal oxides so as to haveimproved stability, have been developed.

Among prior methods of preparing cathode active materials, the mostwidely known methods include a dry calcination method and a wetprecipitation method. According to the dry calcination method, a cathodeactive material is prepared by mixing an oxide or hydroxide of atransition metal such as cobalt (Co) with lithium carbonate or lithiumhydroxide as a lithium source in a dry state, and then calcining themixture at a high temperature of 700-1000° C. for 5-48 hours. The drycalcination method has an advantage in that it is easy to approach,because it is a technology which has been conventionally frequentlyused. However, it has shortcomings in that it is difficult to obtainsingle-phase products because it is difficult to mix raw materialsuniformly, and in the case of multi-component cathode active materialsconsisting of two or more transition metals, it is difficult to arrangetwo or more elements uniformly to the atomic level. Also, in the case ofusing methods of doping or substituting cathode active materials with aspecific metal component in order to improve electrochemicalperformance, there are problems in that the specific metal componentadded in small amounts is difficult to mix uniformly, and the lossthereof necessarily occurs a grinding and classifying process forobtaining granules having the desired size.

Another conventional method for preparing cathode active materials isthe wet precipitation method. According to the wet precipitation method,a cathode active material is prepared by dissolving in water a saltcontaining a transition metal such as cobalt (Co), adding alkali to thesolution to precipitate the transition metal in the form of transitionmetal hydroxide, filtering and drying the precipitate, mixing the driedmaterial with lithium carbonate or lithium hydroxide as a lithium sourcein a dry state, and then calcining the mixture at a high temperature of700-1000° C. for 1-48 hours. The wet precipitation method is known toeasily obtain a uniform mixture by co-precipitating two or moretransition metal elements, but has problems in that it requires a longperiod of time for the precipitation reaction, is performed using acomplicated process, and causes waste acids as by-products. In addition,various methods, including sol-gel methods, hydrothermal methods, spraypyrolysis methods and ion exchange methods, have been suggested asmethods for preparing cathode active materials for lithium secondarybatteries.

Meanwhile, methods of preparing LiCoO₂ particles, and LiMn₂O₄ particles,etc., using supercritical water, have recently been reported (K.Kanamura, et al., Key Engineering Materials, 181-162 (2000), pp.147-150). Japanese Patent Laid-Open Publication No. JP2000-72445Adiscloses a method of preparing a metal oxide for cathode activematerials by allowing lithium ions to react with transition metal ionsin supercritical or subcritical conditions in a batch-type reactor.Also, Japanese Patent Laid-Open Publication No. JP2001-163700 disclosesa method of preparing a metal oxide for cathode active materials byallowing lithium ions to react with transition metal ions insupercritical or subcritical conditions in a batch-type reactor and acontinuous reactor. According to the disclosure of such patentdocuments, in the case of the batch-type reactor, an increase in Li/Coratio, an increase in alkali molar ratio, an increase in nitric acidconcentration and the addition of an oxidizing agent lead to a decreasein the content of impurity Co₃O₄ and an increase in the content ofsingle-phase LiCoO₂. However, particles obtained according to thedisclosure of such patents are not suitable for practical use as cathodeactive materials, because the purity of LiCoO₂ in the particles is onlya maximum of 97.8%. Also, in the case of using the continuous reactor, ametal oxide for cathode active materials is synthesized by continuouslypumping an aqueous cobalt salt solution or an aqueous manganese saltsolution under pressure into the reactor, adding supercritical water andhydrogen peroxide (H₂O₂) thereto, and then allowing the mixture to reactin conditions of about 400° C. and about 300 bar. In this case, thereaction time is as relatively short as 30 seconds or less, but thesynthesized product is known to have low purity and poor electrochemicalproperties. Also, when the above-described methods are used to preparesingle metal oxide such as lithium cobaltate or lithium manganate, theywill provide highly crystalline particles having a size as large assubmicrons (μm). However, these methods have problems in that, when theyare used to prepare a multicomponent (more two components) metal oxide,they cannot synthesize crystals having excellent solid-solutionstability because the crystallization rates of the components aredifferent from each other, and also the synthesized particles aredifficult to apply as cathode active materials because such particlesare as excessively small as the nanometer scale. Thus, there is anurgent need to develop a novel cathode active material, which satisfieshigh performance and low cost requirements, and a preparation methodthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of a preparation process accordingto the present invention.

FIG. 2 is a SEM photograph of a lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized according to Example 1. (a):after drying; and (b): after calcination.

FIG. 3 is an XRD graph of a lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized according to Example 1.

FIG. 4 is an SEM photograph of a lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized according to Example 2. (a):after drying; and (b): after calcination.

FIG. 5 is an XRD graph of a lithium-metal composite oxide synthesizedaccording to Example 2.

FIGS. 6( a) to 6(c) are SEM photographs of lithium-metal compositeoxides (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂) synthesized according to Examples3(a) to 3(c), respectively.

FIGS. 7( a) to 7(c) are XRD graphs of lithium-metal composite oxidessynthesized according to Example 4(a) to 4(c), respectively.

FIG. 8( a) is an FESEM photograph of a lithium metal composite oxide(LiNi_(1/2)Mn_(1/2)O₂) synthesized in Example 5, and FIG. 8( b) is anXRD graph of the composite oxide synthesized in Example 5.

FIG. 9 is an XRD image of a lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized in Example 8. (a): afterdrying; and (b): after calcination.

FIG. 10( a) is an FESEM photograph of a lithium metal composite oxide(LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂) synthesized in Comparative Example 1, andFIG. 10( b) is an XRD graph of the composite oxide synthesized inComparative Example 1.

FIG. 11( a) is an FESEM photograph of a lithium metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized in Example 9, and FIG. 11( b)is an XRD graph of the composite oxide synthesized in Example 9.

FIG. 12 is a graphic diagram showing the results of ⁷Li-NMR analysis oflithium-metal composite oxides (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂)synthesized in Example 1, Example 3(c) and Comparative Example 1.

FIG. 13 is a charge/discharge graph of a lithium secondary battery,which comprises, as a cathode active material, a lithium-metal compositeoxide (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂) synthesized in Example 3(c).

FIG. 14 is a graphic diagram showing the comparison of ratecharacteristics between lithium secondary batteries, which comprise, asa cathode active material, a lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) synthesized in each of Examples 1 and3(c) and Comparative Example 2.

FIG. 15 is a graphic diagram showing the cycle life characteristics oflithium secondary batteries, which comprise, as a cathode activematerial, a lithium-metal composite oxide (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂)synthesized in each of Examples 1 and 3(c).

FIG. 16 is a graphic diagram showing the cycle life characteristics oflithium secondary batteries, which comprise, as a cathode activematerial, a lithium-metal composite oxide (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂)synthesized in each of Comparative Example 1 and Example 9.

FIG. 17 is an XRD graph of a lithium-metal composite oxide (LiFePO₄)synthesized in Example 7.

FIG. 18 is an SEM photograph of a lithium-metal composite oxide(LiFePO₄) synthesized in Example 7.

DISCLOSURE OF THE INVENTION

The present inventors have found that, when a novel preparation methodcomprising either calcining or granulating and then calcining alithium-metal composite oxide synthesized using subcritical water orsupercritical water according to a hydrothermal synthesis method isperformed, the resulting lithium-metal composite oxide has a particlesize larger than that of a multicomponent metal oxide synthesized usingsubcritical or supercritical water according to the prior method, andthus can be used as a multicomponent cathode active material, and alsoit is possible to form a solid solution more uniform than that preparedusing the prior dry calcination method or wet precipitation method. Inparticular, it could be seen that the lithium-metal composite oxideshows crystal stability and excellent physical properties as a result ofan improvement in the ordering of metals, and thus can provide a batteryhaving high capacity characteristics, long cycle life characteristicsand improved rate characteristics.

Accordingly, it is an object of the present invention to provide a novelmethod for preparing the above-described lithium-metal composite oxide,an electrode comprising the lithium-metal composite oxide preparedaccording to said method, and an electrochemical device comprising saidelectrode.

To achieve the above object, in one aspect, the present inventionprovides a method for preparing a lithium-metal composite oxide, themethod comprising the steps of: (a) mixing an aqueous solution of one ormore transition metal-containing precursor compounds with an alkalifyingagent and a lithium precursor compound to precipitate an hydroxide ofthe transition metal; (b) mixing the mixture of step (a) with waterunder supercritical or subcritical conditions to synthesize alithium-metal composite oxide, and drying the lithium-metal compositeoxide; and (c) subjecting the dried lithium-metal composite oxide eitherto calcination or to granulation and then calcination.

In another aspect, the present invention provides an electrodecomprising the lithium-metal composite oxide prepared according to saidmethod.

In still another aspect, the present invention provides anelectrochemical device, preferably a lithium secondary battery,comprising said electrode.

Hereinafter, the present invention will be described in detail.

In the hydrothermal synthesis method that uses supercritical orsubcritical water, a crystal structure having reduced solid-solutionstability because the crystallization rates of the components of amulticomponent lithium-metal composite oxide are different from eachother, and also the synthesized particles are difficult to apply ascathode active materials because such particles are as excessively smallas the nanometer scale. In fact, if an electrode is formed only ofprimary particles (before granulation) produced according to the priorhydrothermal synthesis method, there will be problems in that electrodedensity is insufficient, and capacity per volume and capacity per massare very reduced (see Table 1).

Accordingly, in the present invention, a solid solution, which is moreuniform than that prepared according to the dry calcination method orthe wet precipitation method, can be easily formed by forming a uniformtransition metal precipitate, and then mixing the precipitate withhigh-temperature and high-pressure supercritical or subcritical water,thus synthesizing lithium-metal composite oxide ultrafine particles inwhich lithium ions are inserted into the previously precipitatedtransition metal hydroxide. Also, the synthesized composite oxide fineparticles can be stably grown while the adhesion between the particlecrystals can be increased by either calcining or granulating and thencalcining the particles, instead of simply drying the particles beforeuse. Thus, the lithium-metal composite oxide prepared according to thepresent invention has not only a general granule size that can be usedas an electrode active material, for example, a granule size of 0.1-100μm, but also crystallographic stability and excellent electrochemicalproperties as a result of forming the uniform solid solution andimproving the ordering of metal ions.

Particularly, when both the granulation process and the calcinationprocess are performed, a stable crystal structure can be formed byachieving the growth and sintering of crystals and at the same time, alithium-metal composite oxide having high density (tap density) can beobtained.

In fact, it was seen that the lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) prepared according to the novelpreparation method of the present invention, and the lithium-metalcomposite oxide (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) prepared according to theprior co-precipitation method, had different Li-NMR analysis data (seeFIG. 12), and also showed slightly different battery characteristicswhen they were used in batteries (see FIGS. 13 and 14). Thisdemonstrates that, unlike the lithium-metal composite oxide preparedaccording to the prior method, the lithium-metal composite oxideprepared according to the novel preparation method shows high capacity,high density and improved rate characteristics, because it has ametal-ordered structure in which the components thereof are uniformlydistributed.

Also, the prior wet precipitation method such as the co-precipitationmethod requires at least 12-48 hours up to the production of alithium-metal composite oxide precursor and also requires a sufficientheat treatment time for the diffusion of Li and the production ofcrystals, because the precursor contains no lithium component. Incomparison with this, in the present invention, the lithium-metalcomposite oxide precursor can be formed within a few seconds at hightemperature and high pressure. Moreover, a separate mixing process isnot required, because the precursor already contains the lithiumcomponent before heat treatment. In addition, the heat treatment timefor the diffusion of lithium and the production of a final activematerial can be shortened to promote an increase in the crystallinity ofthe lithium-metal composite oxide. In particular, when a material havingsmall particle size, such as an olivine crystal structure-typelithium-metal composite oxide, is required, fine particles can be easilyproduced without carrying out the granulation process.

Furthermore, in the present invention, the decomposition of byproductnitric acid occurs in supercritical reaction conditions, makingwastewater treatment easy, because the lithium-metal composite oxide issynthesized using an alkalifying agent and a lithium precursor compoundin supercritical or subcritical conditions, unlike the priorhydrothermal synthesis method and supercritical water hydrothermalmethod, which use expensive oxidizing agents.

The lithium-metal composite oxide of the present invention can beprepared on the basis of the hydrothermal synthesis method known in theart, which uses supercritical or subcritical water.

In a preferred embodiment of the present invention, the lithium-metalcomposite oxide can be prepared according a method comprising the stepsof: (a) mixing an aqueous solution of one or more transitionmetal-containing precursor compounds with an alkalifying agent and alithium precursor compound to precipitate a hydroxide of the transitionmetal; (b) mixing the mixture of step (a) with water under supercriticalor subcritical conditions to synthesize a lithium-metal composite oxide;and (c) subjecting the lithium-metal composite oxide either tocalcination or to granulation and then calcination. Herein, in order todistinguish the mixing processes in the step (a) and (b) from eachother, the mixing processes will now be described as primary mixing andsecondary mixing for convenience.

1) Step of mixing an aqueous solution of one or more transitionmetal-containing precursor compounds with an alkalifying agent and alithium precursor compound

This step is a step required for synthesizing a lithium-metal compositeoxide having a uniform composition. In this step, hydroxides oftransition metals other than lithium are precipitated in the form ofultrafine particles.

The transition metal-containing precursor compounds are not specificallylimited as long as they are ionizable compound which contain transitionmetals. Preferred are water-soluble compounds. Herein, the transitionmetals preferably consist of a combination of paramagnetic metals (e.g.,Ni, Mn, etc.) and diamagnetic metals (e.g., Co). Non-limiting examplesof the transition metal precursor compounds include alkoxides, nitrates,acetates, halides, hydroxides, oxides, carbonates, oxlates or sulfatesor combinations thereof, which all contain transition metals.Particularly, preferred are nitrates, sulfates, or acetates. Also, it ispossible to use compounds containing either at least one of the abovetransition metals or combinations thereof, for example, Ni—Mn, Ni—Co,Ni—Mn—Co, etc.

The alkalifying agent serves to provide conditions in which one or moretransition metal compounds are easily hydrolyzed and precipitated ashydroxides. The alkalifying agent is not specifically limited as long asit makes the reaction solution alkaline. Non-limiting examples of thealkalifying agent include alkali metal hydroxides (NaOH, KOH, etc.),alkaline earth metal hydroxides (Ca(OH)₂, Mg(OH)₂, etc.), ammoniacompounds (ammonia water, ammonium nitrate, etc.), and mixtures thereof.Particularly preferred is a case where the metal compound is nitrate,and the alkalifying agent is an ammonia compound. This is becausenitrate ions produced as byproducts are mostly decomposed in the sameprocess, and the remaining ions are also easily removed by washing,drying or calcination in subsequent processes.

As the lithium precursor compound, any compound can be used without anyparticular limitation as long as it is a water-soluble salt thatcontains lithium and is ionizable. Non-limiting examples thereof includelithium nitrate, lithium acetate, lithium hydroxide, and lithiumsulfate. Particularly, lithium hydroxide is preferred, because it servesnot only as a lithium source, but also to increase alkalinity.

The process of mixing the alkalifying agent and the lithium precursorcompound can be performed by mixing both the alkalifying agent and thelithium precursor solution with water. Alternatively, it can beperformed by mixing the alkalifying agent with water and then adding thelithium precursor compound thereto. Alternatively, it can be performedby mixing the alkalifying agent with the lithium precursor compound andthen adding the mixture to water.

In the mixing process of step (a), the transition metal hydroxidesshould be precipitated in the form of fine particles, whereas thelithium hydroxide should be present in a state dissolved in aqueoussolution. Thus, the temperature and pressure in the mixing processshould avoid subcritical or supercritical conditions, such that theprecipitation of the lithium compound does not occur.

(b) Step of adding water under supercritical or subcritical conditionsto the mixture of step (a) and performing secondary mixing

In this step, lithium ions present in the aqueous solution react withsupercritical or subcritical water, so that ultrafine particle crystalsof lithium-metal composite oxide are synthesized, in which lithium ionsare inserted into the precipitated transition metal fine particles.

In the process of step (b), the reaction pressure and temperature shouldbe suitable either for allowing the metal hydroxide precipitate producedin the step (a) to react with lithium ions in the aqueous solution, orallowing lithium ions in the aqueous solution to precipitate ashydroxides. For reference, hydroxides of alkali metals, such as lithium,sodium and potassium, have high solubility in water at ambienttemperature and atmospheric pressure, but when the density of water isdecreased due to high-temperature and high-pressure conditions, thehydroxides show a marked decrease in the solubility thereof in water.For example, the solubility of KOH in water is 2.6 mol (145.8 g/100 gwater) in conditions of ambient temperature, atmospheric pressure waterdensity of 1.0 g/cm′, but is decreased to 300 ppm in conditions oftemperature of 424° C., water density of 0.139 g/cm′ and pressure of 262bar (W. T. Wofford, P. C. Dell'Orco and E. F. Gloyna, J. Chem. Eng.Data, 1995, 40, 968-973). Accordingly, in order to significantly reducethe solubility of the lithium hydroxide, and thus to promote a reactionfor synthesizing a lithium-metal composite oxide, supercritical orsubcritical water should be added and mixed. As used herein, the term“supercritical or subcritical water means high-temperature andhigh-pressure water having a pressure of 180-550 bar and a temperatureof 200-700° C. When the precipitated transition metal hydroxides and thelithium aqueous solution are instantaneously mixed with high-temperaturewater, the temperature of the mixture will rapidly increase from ambienttemperature to subcritical or supercritical temperatures. It is requiredto continuously maintain supercritical or subcritical conditions evenafter adding supercritical or subcritical water.

As described above, the lithium ions subjected to the secondary mixingprocess meet high-temperature and high-pressure water, so that they aresynthesized into lithium-metal composite oxide or precipitated as finelithium hydroxide. Because the precipitated lithium hydroxide has littlechance to contact with the transition metal hydroxides, some thereof donot participate in the reaction, are discharged in conditions of ambientpressure and atmospheric pressure, are dissolved again in the dischargedsolution at ambient temperature and atmospheric pressure, and are wastedin the form of an aqueous solution. For this reason, lithium should beadded in an excess amount in consideration of an amount which isdischarged without participating in the reaction. To satisfy theabove-described amount of lithium, the molar ratio of Li to transitionmetals (e.g., Ni+Mn+Co) is preferably 1.0-20, and more preferably1.0-10. If the molar ratio of Li/(Ni+Mn+Co) is excessively low, lithiumwill participate in the reaction in an amount smaller than thestoichiometric ratio for forming a lithium-metal composite oxide, andthus impurities such as transition metal oxides unreacted with lithium,for example, cobalt oxide, cobalt oxide or manganese oxide, will occur,thus reducing the purity of the desired material. If the ratio isexcessively high, Li in excess of the stoichiometric ratio will remainand should be recovered or wasted from a discharged solution, leading toa decrease in economic efficiency.

The alkali equivalent ratio of the mixture of step (b) is preferably1-10, but the scope of the present invention is not limited thereto. Asused herein, the term “alkali equivalent ratio” is defined as the ratioof the number of equivalents of hydroxyl ions coming from thealkalifying agent (e.g., ammonia water) and LiOH to the number ofequivalents of acidic groups (NO₃, SO₄) coming from the transition metalprecursor compounds (e.g., Co(NO₃)₂, Ni(SO₄), Mn(NO₃)₂, etc.) and thelithium precursor compound (e.g., LiNO₃). For example, it is defined as([NH₃]+[OH])/([NO₃]+2[SO₄]). If the alkali equivalent ratio isexcessively low, the product will contain impurities (e.g., CO₃O₄), andif it is excessively high, the alkaline content of waste water will beexcessively increased.

In the preparation method according to the present invention, the mixingprocess of step (a) is preferably carried out in a mixer (a mixer 1 inFIG. 1; a first mixer), and more preferably a continuous mixer. Also,the second mixing step is preferably carried out in a continuous mixer(a mixer 2 in FIG. 2; a second mixer) such as a tube-type mixer. Thus, auniform precipitate of transition metal hydroxides is formed in thefirst mixer (the mixer 1 in FIG. 1), and when the lithium hydroxidepresent in the mixture in the first mixer is mixed and reacts withsupercritical or subcritical water in the continuous-type second mixer(the mixer 2 in FIG. 2; the second mixer) and a reactor, which areconnected with the first mixer, a lithium-metal composite oxide issynthesized, in which lithium ions are inserted into the previouslyprecipitated transition metal hydroxides.

(C) Step of drying the obtained lithium-metal composite oxide and thensubjecting the dried oxide to calcination or granulation/calcination

The lithium-metal composite oxide produced in the step (b) is very finesuch that it is not suitable for use as a cathode active material inlithium secondary batteries. Thus, this step is performed to makegranules having a size suitable for use as a cathode active material.For reference, it is known that a cathode active for lithium secondarybatteries preferably has a granule size of about 0.1-100 μM (mediumvalue: 2-20 μm).

The granulation process can be generally carried out simultaneously withdrying using various methods known in the art, including a spray dryingmethod, a fluidized bed drying method and a vibration drying method. Thespray drying method is particularly preferred, because it can increasethe tap density of granules through the preparation of spheres. Beforethe concentrate is dried and at the same time, granulated, it can bewashed with clean water to remove impurity salts (e.g., NH₄NO₃ salt),ionic impurities (e.g., NO₃ ⁻ and SO₄ ²⁻ decomposed from nickelcompounds, manganese compounds and cobalt compounds), which can remainin the concentrate.

The calcination process acts to grow lithium-metal oxide particles andto increase the adhesion between the particles. Thus, the calcinationprocess can be carried out either immediately after the step (b) or thegranulation process. It is particularly preferable to carry out thegranulation process and then the calcination process, because in thiscase the primary particles of the granules grow and at the same time,are provided with crystallographic stability. If the calcination processis not performed, crystals will not be stabilized, leading to a greatdeterioration in the initial cycle performance of batteries. Thisdeterioration is the breakdown phenomenon of an unstabilized surface,which frequently appears in LT-LiCoO₂, etc. Also, if the calcinationprocess is not performed, the electrode active material will have largespecific surface area, low tap density, and thus low capacity pervolume.

The calcination temperature is preferably in the range of 600-1200° C.,but there is no particular limitation on the calcination temperature. Ifthe calcination temperature is lower than 600° C., the growth of theprimary particles will not be sufficient, the sintering between theprimary particles will not substantially occur, and thus the primaryparticles will have large specific surface area and low tap density. Inaddition, the growth of the crystals will be insufficient, and thecomposite oxide is not sufficiently stabilized, leading to a reductionin the cycle characteristics of batteries. If the calcinationtemperature is higher than 1200° C., the sintering between the primaryparticles will be excessive, thus reducing the performance of theparticles as cathode active materials.

Before, after or during any step of the steps (a) to (c), it is possibleto add at least one additive selected from among a binder, a sinteringaid, a doping agent, a coating agent, a reducing agent, an oxidizingagent, acid, carbon or a carbon precursor, metal oxide and a lithiumcompound. Particularly, the preparation of a lithium-metal compositeoxide having an olivine-type crystal structure, for example, LiFePO₄,can be prepared through suitable use of phosphoric acid, carbon or acarbon precursor, sucrose, etc., during the preparation process thereof.

The binder can be used to make the granules spherical and to improveparticle size distribution, and non-limiting examples thereof includewater, ammonia water, PVA (polyvinyl alcohol), and mixtures thereof. Thesintering agent can be used during the high-temperature calcination ofthe granules to reduce the calcination temperature or to increase thecalcination density, and non-limiting examples thereof include metaloxides, such as alumina, B₂O₃ and MgO, or precursors thereof, and Licompounds such as LiF, LiOH and LiCO₃. The doping agent and the coatingagent are used to coat the outer surface of electrode active materialcrystals with metal oxide ultrafine particles in order to increase thedurability of the calcined material, when the calcined material is usedin batteries. Non-limiting examples thereof include metal oxides, suchas alumina, zirconia, titania and magnesia, or precursors thereof.

A lithium-metal composite compound having an olivine-type crystalstructure can be doped with a metal such as Mn in order to increase theelectrical conductivity thereof. The oxidizing agent or the reducingagent can be used to control each of the steps to a reducing oroxidative atmosphere. Non-limiting examples of the reducing agentinclude hydrazine, oxalic acid, sucrose, fructose, ascorbic acid,Vitamin C, hydrogen, carbon, hydrocarbon, or mixtures thereof.Non-limiting examples of the oxidizing agent include oxygen, hydrogenperoxide, ozone, or mixtures thereof. The acid is used in the form of areaction material, such as a phosphoric acid compound or a sulfuric acidcompound. Non-limiting examples of the acid include phosphoric acid,sulfuric acid or a mixture thereof. The carbon or the carbon precursorcan be used to increase the electrical conductivity of the preparedmaterial by coating the surface of the material with it or to provide areducing atmosphere. It is particularly useful for a lithium-metalcomposite compound having an olivine-type crystal structure. The lithiumcompound can participate in the reaction during the calcination step toincrease the “a” value of a synthesized lithium-metal composite oxide,for example, Li_(1+a)[Ni_(x)Mn_(y)CO_(z)]M_(b)O_(2−b), and non-limitingexamples thereof include Li compounds, such as LiF, LiOH, LiNO₃ andLiCO₃.

In another aspect, the present invention provides lithium-metalcomposite oxides prepared according to the above-described preparationmethod.

When the preparation method according to the present invention is usedto prepare a multicomponent metal composite oxide comprising at leasttwo metal components in addition to lithium, the metal components can beuniformly mixed and arranged in the crystal structure of the compositeoxide, thus showing an improved ordered structure.

A change in physical properties, such as metal ordering in the crystalstructure of the lithium-metal composite oxide, can be observed byLi-NMR (see FIG. 12 and Table 3).

In Li-NMR, when a strong magnetic field is externally applied to alithium-containing material, chemical shift values will be shifted dueto various interactions between a lithium nucleus having a magneticmoment and the unpaired electrons of components contained in thelithium-containing material, and the structural characteristics (e.g.,clustering, metal ordering) of a specific metal in the crystal structureof the lithium-containing material can be assessed by measuring variouschanges, such as peak intensity and line width, caused by such chemicalshift values.

The present invention can provide a multicomponent (binary or higher)lithium metal composite oxide comprising paramagnetic and diamagneticmetals, the composite oxide satisfying any one of the followingconditions: (1) the ratio of intensity between a main peak of 0±10 ppm(I_(0ppm)) and a main peak of 240±140 ppm (I_(240ppm)),(I_(0ppm)/I_(240ppm)), is less than 0.117·Z wherein Z is the ratio ofmoles of the diamagnetic metal to moles of lithium; (2) the ratio ofline width between the main peak of 0±10 ppm (I_(0ppm)) and the mainpeak of 240±140 ppm (I_(240ppm)), (W_(240ppm)/W_(0ppm)), is less than21.45; and (3) both the conditions (a) and (b), the peaks being obtainedaccording to ⁷Li-NMR measurement conditions and means as describedbelow.

Preferably, the multicomponent lithium-metal composite oxide accordingto the present invention satisfies any one of the following conditions:(1) the ratio of intensity between the main peak of 0±10 ppm (I_(0ppm))and the main peak of 240±140 ppm (I_(240ppm)), (I_(0ppm)/I_(240ppm)), isless than 0.039; (2) the ratio of line width between the main peak of0±10 ppm (I_(0ppm)) and the main peak of 240±140 ppm (I_(240 ppm)),(W_(240ppm)/W_(0ppm)), is 20.30 or less; and (3) both the conditions (a)and (b). More preferably, the multicomponent lithium-metal compositeoxide according to the present invention satisfies any one of thefollowing conditions: (1) the ratio of intensity between the main peakof 0±10 ppm (I_(0ppm)) and the main peak of 240±140 ppm (I_(240ppm)),(I_(0ppm)/I_(240ppm)), is 0.021 or less; (2) the ratio of line widthbetween the main peak of 0±10 ppm (I_(0ppm)) and the main peak of240±140 ppm (I_(240ppm)), (W_(240ppm)/W_(0ppm)), is 15.31 or less; and(3) both the conditions (a) and (b).

[Measurement Conditions]

300 MHz solid state NMR system;

MAS spinning rate: 32 kHz;

Spectral frequency: 116.6420 MHz;

Temperature: room temperature (25° C.);

Chemical shift value standard: 1M LiCl in H₂O;

Pulse sequence: spin echo (90°−τ1−180°−τ2);

Spectrum width: 500,000 Hz;

Pulse length: 90° pulse length −2.25 μsec, and 180° pulse length −4.50μsec;

Dwell time (τ1): 31.25 μsec; and

Pulse delay: 2 sec.

As the paramagnetic and diamagnetic metals, metals known in the art canbe used in the present invention without any particular limitation onthe components or contents thereof as long as they show paramagnetic anddiamagnetic properties. As used herein, the term “paramagnetic metal”refers to a metal having unpaired electrons in the atom, and the term“diamagnetic metal” refers to a metal in which all electrons in the atomare paired. Non-limiting examples of the paramagnetic metal includenickel (Ni), manganese (Mn), and combinations thereof, and non-limitingexamples of the diamagnetic metals include cobalt (Co).

A lithium-metal composite oxide, which can be provided according to thepresent invention, can be represented by any one of the followingformulas 1 to 4, but the scope of the present invention is not limitedthereto:Li_(1+a)A_(1−x)C_(x)O_(2−b)X_(b)(−0.5≦a≦+0.5, 0≦b≦+0.1,0≦x≦+0.1)  [Formula 1];Li₁₊A_(x)B_(2−x−y)C_(y)O_(4−b)X_(b)(−0.5≦a≦+0.5, 0≦b≦+0.1, 0≦x≦+2,0≦y≦0.1)  [Formula 2];Li_(1+a)A_(1−x)C_(x)(YO_(4−b)X_(b))(−0.5≦a≦+0.5, 0≦b≦+0.1,0≦x≦+0.1)  [Formula 3];Li_(1+a)A_(2−x)C_(x)(YO_(4−b)X_(b))₃(−0.5≦a≦+0.5, 0≦b≦+0.1,0≦x≦+0.1)  [Formula 4],wherein A is at least one element selected from among transition metalshaving a six-coordinate structure;B is at least one element selected from among transition metals having afour-coordinate structure;C is at least one element selected from the group consisting of alkalineearth metals and Group 3B elements;X is at least one element selected from the group consisting of Groups5B, 6B and 7B elements; andY is at least one element selected from metalloids or metals having afour-coordinate structure.

In the lithium-metal composite oxide of the present invention, it iseasy to mix elements with each other, and thus it is easy to perform thesubstitution and addition of other elements for improvingelectrochemical properties. For example, the transition metal site inthe active materials represented by the formulas 1 to 4 can be dopedwith trace amounts of other elements selected from among alkaline earthmetals and Group 3B elements. Also, the oxygen site in the activematerials can be easily substituted with an element selected from amongGroups 5B, 6B and 7B, having strong electron affinity.

With respect to the transition metals, those having a six-coordinatestructure are generally stable, but in a spinel structure such as theabove formula 2 may have four-coordinate and six-coordinate structures.Thus, in the above formula 1, A having a six-coordinate structure may beNi, Mn or Co, and in the above formula 2, B having a four-coordinatestructure and A having a six-coordinate structure may be Ni, Mn or Co.Also, in the formula 3 or 4, A having a six-coordinate structure may beFe, Mn, Co, Ni or V, and Y having a four-coordinate structure may be P,Ti, V or Si. Moreover, in the formulas 1 to 4, C may be Al or Mg, and Xmay be F, S or N. However, the scope of the present invention is notlimited thereto.

In the above formulas 1 to 4, the range of “a” is −0.5≦a≦+0.5. If “a” isless than −0.5, the crystallinity of the resulting oxide will not besufficient, and if “a” exceeds 0.5, excess Li will be present in theresulting oxide to form impurities such as Li₂CO₃, which can deterioratethe performance and stability of batteries.

Non-limiting examples of the above-described lithium-metal compositeoxide include LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, LiNi_(1/2)Mn_(1/2)O₂,LiNi_(0.8)Co_(0.2)O₂, LiNi_(0.8)Al_(0.05)Co_(0.15)O₂,LiNi_(0.5)Mn_(1.5)O₄, LiFePO₄, LiMn₂O₄, LiCoO₂, Li_(1+a)Ni_(x)Co_(y)O₂(−0.5<a<0.5, 0<x<1, 0<y<1), LiM₂O₄ (M=a transition metal, for example,Ni, Mn, Co, Ti, Fe, V, etc.), and LiMPO₄ (M=a transition metal, forexample, Ni, Mn, Co, Ti, Fe, V, etc.). In addition, lithium-metalcomposite oxides known in the art are also within the scope of thepresent invention.

The lithium-metal composite oxide according to the present invention ispreferably in the form of circular granules, but there is no particularlimitation on the form thereof. Herein, the size of the granules ispreferably in the range of 0.1-100 μm, but the scope of the presentinvention is not limited thereto. If the size of the granules is lessthan 0.1 μm, they will have an excessively large specific surface area,which makes not easy to fabricate an electrode, and if the size of thegranules exceeds 100 μm, the thickness uniformity of a thin layer duringthe fabrication of an electrode can be deteriorated, resulting in aninferior battery. It is possible to adjust the shape, size and sizedistribution of the granules within the range where the composite oxideshows structural stability and excellent physical properties (tapdensity, packing density, etc.).

Also, a prior electrode active material generally shows an improvementin the rate characteristics of a battery with a decrease in the grainsize thereof, whereas the lithium-metal composite oxide of the presentinvention exhibits excellent rate characteristics, even though it has agrain size significantly larger than that of the lithium-metal compositeoxide prepared according to the prior co-precipitation method (see Table4 and FIG. 14). Moreover, the lithium-metal composite oxide of thepresent invention has advantages in that it easily improve the ratecharacteristics of a battery by controlling the grain size thereof andthat an increase in the grain size of the grains leads to an increase inthe tap density and packing density thereof (see Tables 2 and 4).Herein, the tap density of the lithium-metal composite oxide of thepresent invention is preferably higher than 1.8 g/cm³, but there is noparticular limitation on the tap density.

In still another aspect, the present invention provides an electrode,preferably a cathode, comprising the above-described lithium-metalcomposite oxide.

The electrode according to the present invention can be fabricatedaccording to any conventional method known in the art. In oneembodiment, the electrode can be manufactured by mixing thelithium-metal composite oxide as an active material for both electrodes,preferably a cathode active material, with a binder, so as to prepare anelectrode slurry, and coating the prepared electrode slurry on a currentcollector. In this case, a conducting agent can optionally be used.

In still another aspect, the present invention provides anelectrochemical device comprising: (a) a cathode comprising theabove-described lithium-metal composite oxide; (b) an anode; (c) aseparator; and (d) an electrolyte.

The electrochemical devices include all devices that performelectrochemical reactions, and specific examples thereof include allkinds of primary and secondary batteries, fuel cells, solar cells, andcapacitors. Among the secondary batteries, preferred are lithiumsecondary batteries, including lithium metal secondary batteries,lithium ion secondary batteries, lithium polymer secondary batteries andlithium ion polymer secondary batteries.

The electrochemical device of the present invention can be fabricatedaccording to any conventional method known in the art. In oneembodiment, the electrochemical device can be fabricated by interposinga porous separator between the cathode and the anode within a batterycase and then injecting the electrolyte into the battery case.

There is no particular limitation on the anode, the electrolyte and theseparator, which are to be applied together with the cathode of thepresent invention, and it is possible to use those which have beenconventionally used in prior electrochemical devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in further derailwith reference to examples and comparative examples. It is to beunderstood, however, that these examples are illustrative only and thescope of the present invention is not limited thereto.

Example 1 1-1: Preparation of Lithium-Metal Composite Oxide Granules(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂)

FIG. 1 schematically shows a process for preparing lithium-metalcomposite oxide granules according to the present invention.

An aqueous solution containing 7.0 parts by weight of cobalt nitrate(Co(NO₃)₂.6H₂O), 7,0 parts by weight of nickel nitrate (Ni(NO₃)₂.6H₂O)and 6.9 parts by weight of manganese nitrate (Mn(NO₃)₂.6H₂O) was pumpedunder pressure at a rate of 8 ml/min in conditions of room temperatureand pressure of 250 bar, and a mixture solution containing 13.2 parts byweight of ammonia (NH₃) water and 12.1 parts by weight of aqueouslithium hydroxide (LiOH) solution was pumped under pressure at a rate of8 ml/min in conditions of room temperature and pressure of 250 bar, suchthat the solutions met each other in a first mixer. Herein, the NH₃/NO₃molar ratio was 1.5, and the Li/(Ni⁺ Mn+Co) molar ratio was 4. To themixture, ultrapure water heated to about 450° C. was pumped underpressure at a rate of 96 ml/min at a pressure of 250 bar, such that itmet the mixture in a second mixer. The resulting mixture was allowed toreside in a reactor at 400 for 7 seconds, and then cooled andconcentrated. The concentrate was dried using a spray dryer at 120° C.while it was granulated. The granules were calcined in an oxidationfurnace at 1000° C. for 6 hours, thus obtaining a lithium-metalcomposite oxide (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂). The specific surfacearea and tap density of the prepared lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) are shown in Table 2 below.

1-2. Fabrication of Cathode and Battery

The lithium-metal composite oxide prepared in Example 1-1 was used as acathode active material. The cathode active material, a conducting agentand a binder were mixed with each other at a weight ratio of 95:2.5:2.5in a solvent, thus preparing a cathode slurry. The cathode slurry wasapplied and dried on an aluminum foil, thus obtaining a cathodeelectrode.

An electrolyte was prepared by dissolving 1 mole of LiPF₆ in a mixedsolvent of EC (ethylene carbonate) and EMC (ethyl methyl carbonate) (1:2(v/v)). A coin-type battery was fabricated using the above-preparedcathode and electrolyte.

Example 2

Lithium-metal composite oxide granules, a cathode comprising thegranules, and a lithium secondary battery comprising the cathode, wereprepared in the same manner as in Example 1, except that the NH₃/NO₃molar ratio was changed from 1.5 to 3.0.

As shown in FIG. 4( a), the spray-dried granules were in the form ofspheres having a size of about 10-30 μm, and as shown in FIG. 4( b),these granules maintained the shape thereof even after calcination.

Examples 3(a) to 3(c)

The concentrate synthesized in Example 1 was washed with clean water toremove the removing ions, and then LiOH was added in an aqueous solutionin each of amounts of 0.0 moles (Example 3(a)), 0.10 moles (Example3(b)) and 0.30 moles (Example 3(c)) per mole of the synthesizedlithium-metal composite oxide. Then, each of the mixtures was driedusing a spray dryer at a temperature of 120° C. and at the same time,granulated. The granules were calcined in an oxidation furnace at 1000°C. for 6 hours, thus obtaining lithium-metal composite oxide granules(Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂).

The dried samples and the calcined samples were analyzed with respect totheir size, shape (SEM), specific surface area (BET) and tap density.FIGS. 6( a) to 6(c) show FESEM images of the spray-dried granules. Asshown in FIGS. 6( a) to 6(c), the size of the primary particles of thegranules was increased with an increase in the amount of addition ofLiOH. Also, the addition of LiOH led to an increase in tap density.

Examples 4(a) to 4(c)

Lithium-metal composite oxide granules (Li[Ni_(1/3)Mn_(1/3)CO_(1/3)]O₂),cathodes comprising the granules, and lithium secondary batteriescomprising each of the cathodes, were prepared in the same manner as inExample 1, except that the residence time in the reactor was changedfrom 7 seconds to each of 12 seconds (Example 4(a)), 30 seconds (Example4(b)) and 60 seconds (Example 4(c)). XRD images of the synthesizedgranules are shown in FIGS. 7( a) to 7(c).

Example 5

Lithium-metal composite oxide granules (Li[Ni_(1/2)Mn_(1/2)]O₂),cathodes comprising the granules, and lithium secondary batteriescomprising each of the cathodes, were prepared in the same manner as inExample 1, except that an aqueous solution containing 10.6 parts byweight of nickel nitrate and 10.4 parts by weight of manganese nitratewas used instead of the aqueous solution containing 7.0 parts by weightof cobalt nitrate, 7,0 parts by weight of nickel nitrate and 6.9 partsby weight of manganese nitrate. Changes in the specific surface area andtap density of the prepared active materials before and after thecalcination are shown in Table 2 below.

Example 6

An aqueous solution containing 20.9 parts by weight of cobalt nitrate(Co(NO₃)₂.6H₂O) was pumped under pressure at a rate of 8 ml/min inconditions of room temperature and pressure of 250 bar, and a mixturesolution containing 13.2 parts by weight of ammonia (NH₃) water and 12.1parts by weight of aqueous lithium hydroxide (LiOH) solution was pumpedunder pressure at a rate of 8 ml/min in conditions of room temperatureand pressure of 250 bar, such that the solutions met each other in afirst mixer. Herein, the NH₃/NO₃ molar ratio was 1.5, and the Li/Comolar ratio was 4. To the mixture, ultrapure water heated to about 450°C. was pumped under pressure at a rate of 96 ml/min at a pressure of 250bar, such that it met the mixture in a second mixer. The resultingmixture was allowed to reside in a reactor at 400° C. for 7 seconds, andthen cooled and concentrated. The concentrate was dried using a spraydryer at 120° C., and then calcined in an oxidation furnace at 800° C.for 6 hours, thus obtaining a lithium-metal composite oxide (LiCoO₂).Also, a cathode comprising the lithium-metal composite oxide (LiCoO₂)and a lithium secondary battery comprising the cathode were fabricated.

Example 7

An aqueous solution containing 21.32 parts by weight of iron sulfate(FeSO₄.7H₂O) and 8.84 parts by weight of phosphoric acid (85 wt %) waspumped under pressure at a rate of 4.8 ml/min in conditions of roomtemperature and pressure of 250 bar, and a mixture solution containing13 wt % of ammonia water (NH₃) and 6.43 wt % of an aqueous lithiumhydroxide solution (LiOH.H₂O) was pumped under pressure at a rate of 4.8ml/min at room temperature, such that the solutions met each other in afirst mixer. At this time, sucrose (C₁₂H₂₂O₁₁) was added to the aqueousiron sulfate solution in an amount of 10 wt % based on the weight ofiron sulfate and fed into the reactor. Herein, the NH₃/SO₄ molar ratiowas 1.0, and the Li/Fe molar ratio was 2.0. To the mixture, ultrapurewater heated to about 450° C. was pumped under pressure at a rate of 96ml/min at a pressure of 250 bar, such that it met the mixture in asecond mixer. The resulting mixture was allowed to reside in a reactorat 405° C. for 7 seconds, and then cooled and concentrated. Theconcentrate was dried using a spray dryer at 120° C., and then calcinedin a nitrogen atmosphere furnace at 600° C. for 12 hours, thus obtaininga lithium-metal composite oxide (LiFePO₄). Also, a cathode comprisingthe lithium-metal composite oxide (LiFePO₄) and a lithium secondarybattery comprising the cathode were fabricated. XRD data and an SEMphotograph of the prepared lithium-metal composite oxide (LiFePO₄) areshown in FIGS. 17 and 18, respectively.

Example 8

Lithium-metal composite oxide granules (Li[Ni_(1/3)Mn_(1/3)CO_(1/3)]O₂),a cathode comprising the granules, and a lithium secondary batterycomprising the cathode, were prepared in the same manner as in Example1, except that the Li/(Ni+Mn+Co) molar ratio was changed from 4 to 1.

FIGS. 9( a) and 9(b) show the characteristic peaks ofLi[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂ after drying and calcination,respectively.

Example 9

Lithium-metal composite oxide granules (Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂),a cathode comprising the granules, and a lithium secondary batterycomprising the cathode, were prepared in the same manner as in Example1, except that the NH₃/NO₃ molar ratio was changed from 1.5 to 3.0, andthe calcination temperature was changed from 1000 to 600° C. An SEMphotograph and XRD data of the prepared lithium-metal composite oxide(Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂) are shown in FIGS. 11( a) and 11(b),respectively.

COMPARITIVE EXAMPLES 1 AND 2 Comparative Example 1

Lithium-metal composite oxide granules (Li[Ni_(1/3)Mn_(1/3)CO_(1/3)]O₂),a cathode comprising the granules, and a lithium secondary batterycomprising the cathode, were prepared in the same manner as in Example1, except that the calcination process was omitted.

As shown in FIG. 10( a), well-developed crystals having a size of lessthan about 0.2 μm were synthesized, but as shown in FIG. 10( b), thecharacteristic peak splitting of Li [Ni_(1/3)Mn_(1/3)CO_(1/3)]O₂ was notclear.

Comparative Example 2 Preparation of Lithium-Metal Composite OxideAccording to Co-Precipitation Method

Cobalt nitrate (Co(NO₃)₂.6H₂O), nickel nitrate (Ni(NO₃)₂.6H₂O) andmanganese nitrate (Mn(NO₃)₂.6H₂O) were dissolved in distilled water atan equivalent ratio of 1:1:1, and the solution was slowly added to thesame equivalent of an aqueous lithium hydroxide (LiOH) solution withstirring. A 10% NaOH aqueous solution was added in portions to themixture solution such that the mixture solution reached a pH of about12. The co-precipitated hydroxide precursor was collected and dried at120° C. for 12 hours to obtain dried particles. The dried particles wereoxidized in an oxidation furnace at 1000° C. for 12 hours. The preparedlithium-metal composite oxide was treated in the same manner as inExample 1-2 and Example 1-3, thus fabricating a cathode and a lithiumsecondary battery comprising the same. The specific surface area and tapdensity of the prepared lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂) are shown in Table 2 below.

Test Example 1 Evaluation of Physical Properties of Lithium-MetalComposite Oxide in Each Preparation Step

A change in physical properties of the lithium-metal composite oxideprepared in each step of the preparation method according to the presentinvention was measured.

Before the granulation step of the preparation process according toExample 1, after granulation step (Comparative Example 1), and after thegranulation and calcination steps (Example 1), the lithium-metalcomposite oxide (LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂) was collected as asample, 2.5 g of each of the samples was placed in a mole having adiameter of 1.5 cm and pressurized with 4000 Pa for 5 minutes. Then, thethickness of each of the samples was measured.

From the test results, it could be seen that the lithium-metal compositeoxide calcined after granulation had a very high packing densitycompared to that of the lithium-metal composite oxide, which was notsubjected to calcination after granulation (see Table 1).

TABLE 1 Lithium-metal composite oxide After granulation, but Aftergranulation Before before calcinations and calcination granulation(Comp. Ex. 1) (Example 1) Thickness 12.4 4.51 3.84 (mm)

Test Example 2 Evaluation of Physical Properties of Lithium-MetalComposite Oxides

In order to evaluate the physical properties of the lithium-metalcomposite oxides prepared according to the present invention, thefollowing analysis was performed.

The lithium-metal composite oxides prepared in Examples 1 to 9 wereused, and the lithium-metal composite oxides prepared in ComparativeExamples 1 and 2 were used as a control group.

2-1: SEM Analysis

Surface analysis was performed using a scanning electron microscope(SEM) and, as a result, it could be seen that the lithium-metalcomposite oxides prepared in Examples 1 to 5 maintained their shapewithout changes even after drying or calcination and had a uniformspherical shape having a size of about 10-30 μm (see FIGS. 2 a, 2 b, 4a, 4 b and 8 a). However, it could be seen that the lithium-metalcomposite oxide prepared in Comparative Example 1 was in the form ofcrystals having a size of less than 0.2 μm (see FIG. 10 a).

2-2: XRD Analysis

The samples were subjected to XRD (x-ray diffraction) analysis. As aresult, it could be seen that the lithium-metal composite oxidesprepared in Examples 1 to 5 showed distinct peak splitting at 2 thetavalues of 30-40° (diffraction lines 006, 102) and 60-70° (diffractionlines 108, 110), and clearly showed the characteristic peaks ofLi[Ni_(1/2)Mn_(1/2)]O₂ (see FIG. 8 b) and the characteristic peaks ofLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ (see FIGS. 3, 5, 7 a, 7 b and 7 c).

However, in the lithium-metal composite oxide prepared in ComparativeExample, the splitting characteristic peak ofLiNi_(1/3)Mn_(1/3)CO_(1/3)O₂ was not clearly shown (see FIG. 10 b).

2-3: Analysis of Tap Density and Specific Surface Area (BET)

Changes in the specific surface area and tap density of thelithium-metal composite oxide before and after calcination were measuredand analyzed. As a result, it could be seen that the lithium-metalcomposite oxides prepared according to the present invention had smallspecific surface area and high tap density after calcination compared tothe lithium-metal composite oxide prepared in Comparative Example 1according to a conventional method known in the art (see Table 2). Thisis believed to be because the spherical granules formed in the dryingstep was subjected to the calcination step, and thus the growth ofcrystals constituting the granules well occurred due to the sinteringbetween the crystals to increase the size of the crystals, and thepacking efficiency of the granules was excellent due to the sphericalshape.

TABLE 2 Examples 3 Comparative Properties Conditions 1 2 a B c 5 Example2 Specific surface Before 9 7.8 9.5 9 8.6 8.5 8.3 area (m²/g)calcination After 0.23 0.31 0.43 0.40 0.23 0.31 0.63 calcination Tapdensity (g/cc) Before 1.8 1.95 1.8 1.85 1.9 1.7 1.3 calcination After2.4 2.4 2.4 2.55 2.6 2.4 2.0 calcination

2-4: Li-NMR Analysis

The lithium-metal composite oxides (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂)prepared in Examples 1 and 3(c) were used, and the lithium-metalcomposite oxide (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) prepared in ComparativeExample 2 according to the co-precipitation method was used as a controlgroup.

The test samples were analyzed in 300 MHz ⁷Li-MAS magic angle spinning)NMR at a spinning rate of 32 kHz, and the analysis results are asfollows.

The lithium-metal composite oxide of Comparative Example 2, preparedaccording to the prior co-precipitation method, showed a sharp peaklocated around 0 ppm and a very broad peak (500-1000 ppm) having thecenter of mass at around 240 ppm (see FIG. 12). In this case, the sharpLi peak (peak A) at around 0 ppm showed a Li peak, around of which onlydiamagnetic transition metal Co³⁺ (t_(2g) ⁶e_(g) ⁰) was coordinated, andthis peak suggests that a Co cluster of Co³⁺ was present in a portion ofthe composite oxide. Also, the significantly broad peak (B) consideredto have the center of mass at around 240 ppm showed an Li peakcoordinated by paramagnetic and diamagnetic metals (e.g., Ni²⁺ (t_(2g)⁶e_(g) ²), Mn⁴⁺ (t_(2g) ⁶) and Co³⁺ (t_(2g) ⁶e_(g) ⁰)) and this suggeststhat chemical shift values are significantly shifted and expanded due tothe interactions between the unpaired electrons of paramagnetic metals(e.g., Ni²⁺ and Mn⁴⁺) and lithium nuclei. As a result, in the priorlithium-metal composite oxide prepared according to the conventionalmethod, chemical shift values are various due to plural interactionsdepending on the orientation between Li nuclei and the unpairedelectrons of paramagnetic metals, and plural peaks having such variouschemical shift values are overlapped to form a significantly broad peak.In short, it is considered that the paramagnetic and diamagnetic metalsof the lithium-metal composite oxide are randomly distributed aroundlithium, rather than present in the oxide itself in a uniformly mixedstate.

In comparison with this, it could be seen that the lithium-metalcomposite oxide of the present invention showed not only a greatdecrease in the intensity of a Li peak around a Co cluster, but also asignificant decrease in the line width of the broadest peak, despitethat it had the same components and composition as those of ComparativeExample 2 (see FIG. 12 and Table 3).

A decrease in the ratio of the intensity of a peak located around 0 ppmto the intensity of a peak located around 240 ppm,(I_(0ppm)/I_(240ppm)), means a decrease in the clustering of diamagneticmetals in the composite oxide, that is, a decrease in Co segregation. Infact, in order to express changes in I_(A)/I_(B) (I_(0ppm)/I_(240ppm))values as numerical values, the overlapped main peaks (A and B) andspinning side bands were separated by fitting according to thedeconvolution method, and the intensity of each of the main peaks wascalculated. The calculation results are shown in Table 3 below. Becausea decrease in I_(A)/I_(B) (I_(0ppm)/I_(240ppm)) value means a decreasein Co segregation, it could be seen that the lithium-metal compositeoxide of the present invention showed a great decrease in the clusteringof a specific metal in crystals, that is, a great decrease in Cosegregation, compared to the lithium-metal composite oxide preparedaccording to the prior method. This suggests that the metal componentsof the lithium-metal composite oxide according to the present inventionwere more uniformly distributed (see FIG. 12 and Table 3).

Also, a decrease in line width between the peaks, (W_(240ppm)/W_(0ppm)),suggests that the ordering of paramagnetic and diamagnetic metals (e.g.,Ni, Mn and Co) in the crystalline structure of the composite oxide wasrelatively improved, and thus the broadening of the Li peak coordinatedwith these metals was decreased. In fact, in order to express, as anumerical value, a visible change in the line width of the broad peak(peak B) located around 240 ppm, the peak located around 240 ppm andspinning side band peaks were separated and fitted according to thedeconvolution method, and then the ratio of the average line width ofthese peaks to the line width of the sharp peak located around 0 ppm wascalculated. Because a decrease in the ratio of line width between thepeaks, (W_(B)/W_(A)), means a decrease in the broadening of the Li peakscoordinated with Ni, Mn and Co, and thus it could be seen that thelithium-metal composite oxide of the present invention showed a decreasein paramagnetic intensity locally interacting with Li ions, compared tothe lithium-metal composite oxide prepared in Comparative Example 2according to the conventional method, and showed a regular arrangementof Ni²⁺/Mn⁴⁺/Co³⁺ in the oxide. This suggests that the metal componentsof the lithium-metal composite oxide according to the present inventionwere more uniformly distributed, leading to an improvement in theordering of metals.

Herein, the ratios of intensity and line width between theabove-described peaks are influenced by magnetic intensity in Li-NMR,MAS spinning rate, the composition and valence of a transition metal,etc., and thus, when the measurement of the ratios is performed inconditions different from the measurement conditions of the presentinvention, results different from those of the present invention can beobtained. For this reason, the ratios of intensity and line widthbetween specific peaks in the present invention are meaningful only inthe conditions suggested in the present invention.

TABLE 3 Comparative Example 2 (NMC) Example 1 Example 3(c) I_(A)/I_(B)0.039 0.021 0.012 W_(B)/W_(A) 21.45 20.30 15.31 I_(A): Intensity of Lipeak around Co cluster; I_(B): Intensity of Li peak around Ni, Mn andCo; W_(A): Line width (Hz) of Li peak around Co cluster, and W_(B):Average line width (Hz) of broad lithium peaks around Ni, Mn and Co.

Test Example 3 Evaluation of Performance of Lithium Secondary Battery

The performance of a lithium secondary battery comprising thelithium-metal composite oxide of the present invention was evaluated inthe following manner.

3-1: Evaluation of Charge/Discharge Capacity

The lithium secondary battery comprising the lithium-metal compositeoxide of the present invention was charged to 4.4 V in constant-currentand constant-voltage (CC/CV) modes and discharged to 3 V in aconstant-current (CC) mode. The rate characteristics of the battery wasobtained by charging and discharging the battery in the followingsequence: 2 cycles at 0.1 C, 2 cycles at 0.2 C, 1 cycle at 0.5 C, and 1cycle at 1 C. Then, the battery was subjected to charge/discharge cyclesat 0.5 C.

From the test results, it could be seen that the lithium secondarybattery of Example 3(c), employing the lithium-metal composite oxidegranules (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) as a cathode active material,showed a charge capacity of 192 mAh/g and a discharge capacity of 176mAh/g, suggesting that the battery could achieve excellent performance(see FIG. 13).

3-2: Evaluation of Cycle Life Characteristics

Lithium secondary batteries that comprise, as a cathode active material,each of the lithium-metal composite oxides(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) prepared in Examples 1, 3 (c) and 9, wereused. The battery of Comparative Example 1, prepared without performingthe calcination process, was used as a control group.

From the test results, it could be seen that the battery of ComparativeExample 1 showed a rapid reduction in the performance thereof within afew charge/discharge cycles (see FIG. 16). However, the lithiumsecondary batteries comprising each of the cathode active materialsprepared in Examples 1 and 3 showed little or no reduction in theperformance thereof even after 20 charge/discharge cycles (see FIG. 15).

3-3: Evaluation of Performance with Conventional Lithium-Metal CompositeOxide

Lithium secondary batteries, which comprise, as a cathode activematerial, each of lithium-metal composite oxides consisting of the samecomponents, were comparatively evaluated with respect to the performancethereof.

For this purpose, lithium secondary batteries, which comprise, as acathode active material, each of the lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) prepared in Examples 1 and 3(c), wereused, and the battery of Comparative Example 2, prepared according tothe co-precipitation method, was used as a control group.

From the test results, it could be seen that the batteries of Examples 1and 3, comprising the inventive lithium-metal composite oxide as acathode active material, had excellent rate characteristics compared tothe battery of Comparative Example 2 (see FIG. 14).

Test Example 4 Examination of Relationship Between Grain Size ofLithium-Metal Composite Oxide and Performance of Lithium SecondaryBattery

In order to examine the relationship between the grain size of thelithium-metal composite oxide prepared according to the presentinvention and the performance of a battery, the following test wasperformed.

Table 4 below shows the grain sizes of the lithium-metal composite oxide(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) of Examples 1 and 3(c) and thelithium-metal composite oxide of Comparative Example 2 (co-precipitationmethod). Herein, the grain sizes were determined by the Scherrer'sequation based on the half width of peaks after XRD measurement.

Generally, a decrease in the grain size of electrode active materialleads to an improvement in the rate characteristics of a battery. Thus,the grain size of each of the lithium-metal composite oxides prepared inthe present invention and the lithium-metal composite oxide preparedaccording to the co-precipitation method (Comparative Example 2) wasmeasured. As a result, it could be seen that, even though thelithium-metal composite oxides of the present invention had a grain sizesignificantly larger than that of the lithium-metal composite oxideprepared according to the co-precipitation method (Comparative Example2), it showed good rate characteristics (see FIG. 14).

Also, the lithium-metal composite oxide according to the presentinvention may have advantages in that it easily improve the ratecharacteristics of a battery by controlling the grain size thereof andthat an increase in the grain size of the grains leads to an increase inthe tap density and packing density thereof (see Table 4). Also, anincrease in packing density leads to an increase in electrode density,thus making it to realize a high-capacity battery.

TABLE 4 Comparative Example 2 Example 1 Example 3 (c) Grain Size (nm)339 668 1000

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, according to the present invention,the lithium-metal composite oxide synthesized based on the priorsupercritical hydrothermal synthesis method is subjected either tocalcination or to granulation and then calcination. Thus, unlike theprior dry calcination method or wet precipitation method, a uniformsolid solution can be formed and the ordering of metals in the compositeoxide can be improved. Accordingly, the lithium-metal composite oxide ofthe present invention has crystal stability and excellent properties,and thus can provide a battery having high capacity and long cycle lifecharacteristics.

Although the preferred embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

The invention claimed is:
 1. A method for preparing a lithium-metalcomposite oxide, the method comprising the steps of: (a) mixing anaqueous solution of one or more transition metal-containing precursorcompounds with an alkalifying agent and a lithium precursor compound, inwhich the aqueous solution is not under supercritical or subcriticalconditions, to precipitate hydroxides of the transition metals; (b)mixing the mixture of step (a) with water under supercritical orsubcritical conditions to synthesize a lithium-metal composite oxide,and drying the lithium-metal composite oxide; and (c) subjecting thedried lithium-metal composite oxide either to calcination or togranulation and then calcination.
 2. The method for preparing a lithiumcomposite oxide of claim 1, wherein the step (c) can grow crystals ofthe lithium-metal composite oxide synthesized in the step (b) and, atthe same time, can increase the adhesion between the crystals.
 3. Themethod for preparing a lithium-metal composite oxide of claim 1, whereinthe transition metal-containing precursor compounds are selected fromthe group consisting of transition metal-containing nitrates, sulfatesand acetates, and the lithium precursor compound is selected from thegroup consisting of lithium hydroxide and lithium nitrate.
 4. The methodfor preparing a lithium-metal composite oxide of claim 1, wherein thealkalifying agent is a compound selected from the group consisting ofalkali metal hydroxides, alkaline earth metal hydroxides and ammoniacompounds.
 5. The method for preparing a lithium-metal composite oxideof claim 1, wherein the molar ratio of lithium to the transition metals(Li/transition metals) in the step (b) is 1.0-20.
 6. The method ofpreparing a lithium-metal composite oxide of claim 1, wherein the alkaliequivalent ratio of the mixture in step (b) is in a range of 1-10. 7.The method of preparing a lithium-metal composite oxide of claim 1,wherein the water under supercritical or subcritical conditions in thestep (b) has a pressure of 180-550 bar and a temperature of 200-700° C.8. The method for preparing a lithium-metal composite oxide of claim 1,which is carried out in a first mixer, a second continuous mixer and areactor connected with the mixers, in which the step (a) is carried outin the first mixer to form a uniform precipitate of the transition metalhydroxides, and lithium hydroxide present in the mixture in the firstmixer is subjected to the step (b) in the second mixer and reactorconnected with the first mixer so as to synthesize the lithium-metalcomposite oxide in which lithium ions are inserted into the precipitatedtransition metal hydroxides.
 9. The method for preparing a lithium-metalcomposite oxide of claim 1, wherein the granulation in the step (c) iscarried out using a method selected from the group consisting of a spraydrying method, a fluidized bed drying method and a vibration dryingmethod.
 10. The method for preparing a lithium-metal composite oxide ofclaim 1, wherein the calcination in the step (c) is carried out at atemperature of 600-1200° C.
 11. The method for preparing a lithium-metalcomposite oxide of claim 1, wherein at least one additive selected fromthe group consisting of a binder, a sintering aid, a doping agent, acoating agent, a reducing agent, an oxidizing agent, an acid, carbon, acarbon precursor, metal oxide and a lithium compound is additionallyused before, after or during any one step of the steps (a) to (c). 12.The method of preparing a lithium-metal composite oxide of claim 11,wherein the additive is a compound containing at least one elementselected from the group consisting of Li, Al, Zr, Mg, Ti, C, P, halogen,and combinations thereof.
 13. The method of preparing a lithium-metalcomposite oxide of claim 1, wherein the alkali equivalent ratio of themixture in step (b) is in a range of 1-10.
 14. A method for preparing alithium-metal composite oxide, the method comprising the steps of: (a)mixing an aqueous solution of one or more transition metal-containingprecursor compounds with an alkalifying agent and a lithium precursorcompound, in which the aqueous solution is not under supercritical orsubcritical conditions, to precipitate hydroxides of the transitionmetals; (b) mixing the mixture of step (a) with water undersupercritical or subcritical conditions to synthesize a lithium-metalcomposite oxide, and drying the lithium-metal composite oxide; and (c)subjecting the dried lithium-metal composite oxide either to calcinationor to granulation and then calcinations; wherein the dried lithium-metalcomposite oxide satisfies any one of the following conditions: (1) theratio of intensity between a main peak of 0±10 ppm (I_(0ppm)) and a mainpeak of 240±140 ppm (I_(240ppm)), (I_(0ppm)/I_(240ppm)), is less than0.117 Z wherein Z is the ratio of moles of the diamagnetic metal tomoles of lithium; (2) the ratio of line width between the main peak of0±10 ppm (I_(0ppm)) and the main peak of 240±140 ppm (I_(240ppm)), (W₂₄₀ppm/W_(0ppm)) is less than 21.45; and (3) both the conditions (a) and(b), the peaks being obtained according to ⁷ Li-NMR measurement.
 15. Themethod for preparing a lithium-metal composite oxide of claim 14,wherein the transition metal-containing precursor compounds are selectedfrom the group consisting of transition metal-containing nitrates,sulfates and acetates, and the lithium precursor compound is selectedfrom the group consisting of lithium hydroxide and lithium nitrate. 16.The method for preparing a lithium-metal composite oxide of claim 14,wherein the alkalifying agent is a compound selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxidesand ammonia compounds.
 17. The method for preparing a lithium-metalcomposite oxide of claim 14, wherein the molar ratio of lithium to thetransition metals (Li/transition metals) in the step (b) is 1.0-20. 18.The method of preparing a lithium-metal composite oxide of claim 14,wherein the water under supercritical or subcritical conditions in thestep (b) has a pressure of 180-550 bar and a temperature of 200-700° C.19. The method for preparing a lithium-metal composite oxide of claim14, wherein the calcination in the step (c) is carried out at atemperature of 600-1200° C.
 20. The method for preparing a lithium-metalcomposite oxide of claim 14, wherein at least one additive selected fromthe group consisting of a binder, a sintering aid, a doping agent, acoating agent, a reducing agent, an oxidizing agent, an acid, carbon, acarbon precursor, metal oxide and a lithium compound is additionallyused before, after or during any one step of the steps (a) to (c).